Displacement analysis and design of a (2-RRU)-URR parallel mechanism performing 2R1T output motion for thumb rehabilitation

Abstract

The thumb assists other fingers, and any damage in its functionality prevents the human hand from performing dexterous functions. In this paper, the kinematic design of the (2-RRU)-URR parallel mechanism as the application of the thumb rehabilitation device is proposed. This mechanism is an over-constrained mechanism capable of achieving the required mobility with fewer joints. Three degrees of freedom exist-two rotational and one translational mobility-that are related to each thumb movement: adduction-abduction and flexion-extension. Considering the narrow space of the hand, actuators are designed to divide its placement into the surface of the palm. To avoid the collisions between the device and the hand, an offset was adopted. The displacement analysis problem is solved by dividing it into two parts: the planar motion generator (PMG) and orientation generator (OG), according to each functional motion, and the corresponding equations and procedures are presented. To clarify the basic characteristics of this mechanism, the reachable workspace of the PMG and rotational ability and sensitivity of the OG is demonstrated numerically. Because a large input torque difference is dangerous in the rehabilitation mechanism, the effective workspace is determined according to the magnitude of the input torque differences and compared with the measured thumb movements.